A Democratic Unionist MP who for two years in a row has claimed the most expenses in the House of Commons is being investigated by a parliamentary watchdog.

Jim Shannon, the MP for Strangford in Northern Ireland, was paid a total of £205,798 in taxpayer-funded expenses last year, excluding travel.

The rare formal investigation by the Independent Parliamentary Standards Authority’s compliance officer, Peter Davis, concerns a claim made by Shannon in relation to travel and subsistence. Davis said no further details of the alleged wrongdoing would be published until the investigation had been concluded.

Shannon was first elected to the Strangford constituency in 2010, replacing the former DUP MP Iris Robinson, who retired from politics after the exposure of her affair with a 19-year-old man. At last year’s general election, Shannon was returned to parliament with a 9,000 majority.

Shannon has previously defended his expenses bill, pointing out that he is one of the busiest parliamentarians in the House of Commons. “I would say that I have one of the best attendance records and workloads in Westminster,” he said. “Last year I was on committees and there for votes, and the flight is the only way to get there.”

Davis has been criticised for settling almost all his cases without any publicity, despite promising to disclose the outcome of all investigations. He failed to announce that claims by three MPs had been referred to police last year.

The officer, who operates separately from Ipsa, was previously forced to back down when he proposed carrying out investigations in secret to prevent “reputational damage” to MPs.

Details of his casework suggest he has been avoiding disclosure by staging in-depth “assessments” of complaints, during which politicians can hand back money or resolve issues without facing formal investigations.

About 40 assessments were undertaken in 2014-15, of which just one – relating to the Tory MP Bob Blackman’s mileage claims – became a formal investigation and so was made public.